Cardiomyocyte-specific role of miR-24 in promoting cell survival.

Cardiomyocyte cell death is a major contributing factor to various cardiovascular diseases and is therefore an important target for the design of therapeutic strategies. More recently, stem cell therapies, such as transplantation of embryonic or induced pluripotent stem (iPS) cell-derived cardiomyocytes, have emerged as a promising alternative therapeutic avenue to treating cardiovascular diseases. Nevertheless, survival of these introduced cells is a serious issue that must be solved before clinical application. We and others have identified a small non-coding RNA, microRNA-24 (miR-24), as a pro-survival molecule that inhibits the apoptosis of cardiomyocytes. However, these earlier studies delivered mimics or inhibitors of miR-24 via viral transduction or chemical transfection, where the observed protective role of miR-24 in cardiomyocytes might have partially resulted from its effect on non-cardiomyocyte cells. To elucidate the cardiomyocyte-specific effects of miR-24 when overexpressed, we developed a genetic model by generating a transgenic mouse line, where miR-24 expression is driven by the cardiac-specific Myh6 promoter. The Myh6-miR-24 transgenic mice did not exhibit apparent difference from their wild-type littermates under normal physiological conditions. However, when the mice were subject to myocardial infarction (MI), the transgenic mice exhibited decreased cardiomyocyte apoptosis, improved cardiac function and reduced scar size post-MI compared to their wild-type littermates. Interestingly, the protective effects observed in our transgenic mice were smaller than those from earlier reported approaches as well as our parallelly performed non-genetic approach, raising the possibility that non-genetic approaches of introducing miR-24 might have been mediated via other cell types than cardiomyocytes, leading to a more dramatic phenotype. In conclusion, our study for the first time directly tests the cardiomyocyte-specific role of miR-24 in the adult heart, and may provide insight to strategy design when considering miRNA-based therapies for cardiovascular diseases.